The invention relates to a record carrier provided with information in an optically readable information structure of track-wise arranged information areas which alternate with intermediate areas.
In the case of a circular disc-shaped record carrier the information tracks may be concentric tracks or alternatively, quasi-concentric tracks which are linked to each other, together constituting a spiral track.
Such a record carrier is described in the Applicant's Netherlands patent application No. 78 03517, which corresponds to U.S. application Ser. No. 419,640 filed Sept. 17, 1982. The known record carrier may contain a television programme, the information being encoded in the frequency and/or the dimensions of the information areas in the track direction. The information areas may comprise pits pressed into the carrier surface or hills projecting from this surface or both. Alternatively, the information may be encoded in digital form, in which case the information areas and intermediate areas may also have a constant dimension in the track direction. A specific combination of information areas and intermediate areas then represents a specific combination of digital zeros and ones.
For optical record carriers a maximum information density is desirable, which in the case of a carrier containing a television programme means: a maximum playing time. For this purpose the information tracks would have to be arranged as closely as possible to each other. However, the distance between the information tracks cannot be selected arbitrarily small. For known record carriers, in which the information areas of adjacent information tracks have the same geometry, except for the dimension in the track direction, all the information areas influence the read-beam radiation in a similar manner. The read spot formed on the information structure by the read beam has a specific, for example Gaussian, intensity distribution. The half-value diameter of said spot, i.e. the distance between two points in the spot where the intensity is equal to 1/e.sup.2 of the intensity in the centre of the spot, is of the order of magnitude of or greater than the width of the information areas, hereinafter referred to as track width. This means that even in the case of a correct tracking of the read spot, an amount of radiation will fall outside the track and may be incident on adjacent tracks. The amount of radiation incident on the adjacent tracks increases as the track distance decreases. The track distance is the distance between the edges of two radially adjacent information areas. This distance is equal to the width of the informationless lands situated between the information tracks. A certain part of the radiation which is incident on the adjacent tracks and which is modulated by the information areas of these information tracks may reach a radiation-sensitive detector, which is adapted to receive the radiation modulated by the track to be read. This effect, the crosstalk effect, determines the minimum distance between the information tracks.
In Netherlands patent application No. 78 03517, which has been laid open to public inspection. It is proposed to increase the information density by giving the information pits of adjacent tracks different depths and reading said information tracks in different ways. The information tracks whose information areas have a first depth are read by determing the variation of the total intensity of the radiation returning from the record carrier and passing through the pupil of the read objectives. This is the so-termed integral read method. The information tracks whose information areas have the second depth are read by determining the intensity difference in two tangentially different halves of the pupil of the read objective. This is the so-called differential read method. When reading a first information track with the one read method an adjacent information track to be read with the other read method is hardly observed, so that the information tracks can be arranged substantially more closely to each other without excessive crosstalk.
For reading such a record carrier a suitably adapted read apparatus should be used, that is to say a read apparatus which can be switched from the one read method to the other. Since the two read methods have different optical transfer functions ("Modulation Transfer Function": "MTF") the alternate use of the two read methods may be perceptible in the signal which is ultimately produced by the read apparatus. Moreover, the information areas with a lower spatial frequency cannot be read in an optimum manner by means of the differential method. Furthermore, the transitions between track portions with deeper information areas and the track portions with shallower information areas should be optically marked, in the record carrier itself, in order to provide an indication to the read apparatus when switching over is to take place. The servo system which ensures that the read spot formed on the information structure accurately follows an information track to be read is sensitive to the depth of the information areas. Therefore, this servo system should also be adapted in the case of a transition from deeper to shallower information areas and vice versa.